1. Field of the Invention
The present invention relates to a main shaft balancing device for a machine tool. In particular, the present invention relates to a main shaft balancing device that supports a main shaft unit with an upright gas spring and that can reduce the load on a means for raising and lowering (vertical displacement) of the main shaft unit.
2. Description of the Related Art
In the related art, machine tools such as machining centers and the like are provided with a main shaft unit comprising a shaft for attaching a tool and an electric motor for rotating this shaft. This main shaft unit is raised and lowered by being supported by a vertical displacement mechanism. This type of vertical displacement mechanism usually comprises a vertical ball screw shaft, a ball screw nut which screws onto the ball screw shaft and is connected to the main shaft unit, and an electric motor. By rotating the ball screw shaft with the electric motor, the main shaft unit is raised and lowered together with the ball screw nut.
However, with this type of machine tool, the heavy main shaft unit must be raised and lowered while being supported by the vertical displacement mechanism. Therefore, there is a risk of having a large load on the vertical displacement mechanism and having a lower precision for the vertical position of the main shaft unit. It is also difficult to increase the speed for raising and lowering of the main shaft unit. In addition, the vertical displacement mechanism which supports the heavy shaft unit becomes large, and the manufacturing costs become high.
A main shaft balancing device has been used to lighten the load on the vertical displacement mechanism of the machine tool. For example, in Japanese Laid-Open Patent No. 6-297217, there is disclosed a main shaft balancing device, comprising: a hydraulic cylinder that is affixed to the upper side of a main shaft unit and has a piston rod extending downwards and has the rod end connecting to the main shaft unit; a directional control valve that is connected to a double oil chamber that is separated by the piston of the hydraulic cylinder; a hydraulic pressure supplying mechanism that supplies hydraulic pressure to the hydraulic cylinder via the directional control valve; and a control device that controls the directional control valve.
With this main shaft balancing device, when raising and lowering the main shaft unit by the vertical displacement mechanism, the piston rod of the hydraulic cylinder extends and contracts in synchrony with the raising and lowering of the main shaft unit. Due to the hydraulic cylinder, an upward force approximately equal to the weight of the main shaft unit is generated, and this can lighten the load on the vertical displacement mechanism. When working a workpiece, when the main shaft unit requires a downward pushing force, the main shaft unit is lowered by the hydraulic cylinder, and the downward pushing force is generated by this driving force.
The present applicant has proposed using a gas spring as the main shaft balancing device. In general, a gas spring has a cylinder body that is filled with compressed gas and a rod that passes through a wall member on the rod side end (rod side end wall member) of the cylinder body. However, because the compressed gas inside the cylinder body gradually leaks out and the gas pressure becomes reduced, compressed gas inside the cylinder body must be periodically replenished.
In order to suppress the leakage of compressed gas inside the cylinder body, a seal structure that seals between the end wall member on the rod side of the cylinder body and the rod that passes through this wall member is particularly important. However, the seal structure of the gas spring of the prior art consisted only of a structure in which there is one or a plurality of ring-shaped seal members provided between the rod side end wall member and the rod. As a result, it is difficult to effectively suppress leakage of compressed gas inside the cylinder body.
With the machine tool of the prior art, as described above, ones without a main shaft balancing device must raise and lower the heavy main shaft unit by the vertical displacement mechanism. As a result, the load on the vertical displacement mechanism is large, and the positional precision in the vertical direction of the main shaft unit by the vertical displacement mechanism, or in other words the machining precision, is reduced. It is difficult to increase the speed for raising and lowering the main shaft unit. The vertical displacement mechanism that supports the heavy main shaft unit becomes large, and manufacturing costs are high.
With the main shaft balancing device of Japanese Laid-Open Patent Publication No. 6-297217, when raising and lowering the main shaft unit with the vertical displacement mechanism, it is difficult to control the directional control valve with the control device such that an upward force that is approximately equal to the weight of the main shaft unit is generated by the hydraulic cylinder. As a result, the main shaft unit is limited by the hydraulic cylinder, and raising and lowering at high speeds is difficult. Because a directional control valve and a hydraulic pressure supplying mechanism and a control device and the like must be provided in addition to the hydraulic cylinder, the structure is complex, and the manufacturing costs are high.
On the other hand, when a gas spring is used as the main shaft balancing device as proposed by the present inventor, in order to support an especially heavy main shaft unit, the gas pressure of the compressed gas inside the cylinder body may be set very high. In this situation, as described above, with only ring-shaped sealing members for the seal structure that seals between the rod side end wall member of the cylinder body and the rod of the gas spring, it is difficult to prevent with certainty the leakage of compressed gas inside the cylinder body.
For example, on the surface of the rod of the gas spring, there may be polishing scratches or scratches from foreign objects or there maybe embedding of foreign objects. In these situations, when the vertical displacement of the main shaft unit is stopped or in other words when the extension and contraction of the rod is stopped, if the position of the scratches or foreign objects coincides with the position of the ring-shaped seal member, the compressed gas inside the cylinder body may leak to the outside through the spaces formed by these scratches or foreign objects. In this manner, with the main shaft balancing device using a gas spring of the prior art, it is difficult to ensure a tight seal between the rod side end wall member of the cylinder body and the rod.
Furthermore, with the main shaft balancing device using the gas spring of the prior art, it is difficult to supply lubricating oil to between the rod side end wall member of the cylinder body and rod or to the ring-shaped seal members. As a result, it is difficult to have good lubrication in the sliding parts between the rod end wall member with the ring-shaped seal member and the rod, and there can be abrasion of these members. When there is abrasion of these members, leakage of the compressed gas inside the cylinder body from between the rod end wall member and the rod occurs more readily.